libc++abi: build with -fvisibility=hidden

[compnerd]

Summary

This enables building libc++abi with -fvisibility=hidden if the compiler supports it. This is required to ensure that only public interfaces are made available from the library. Currently, the ABI difference between the default and hidden versions is:

• __cxa_new_handler
• __cxa_terminate_handler
• __cxa_unexpected_handler

On Darwin, the difference between the version shipped in 10.11 and the hidden visibility is a bit more interesting as follows:

• __cxa_new_handler
• __cxa_terminate_handler
• cxa_unexpected_handler + cxa_throw_bad_array_new_length + __cxa_uncaught_exceptions

Linux is even more interesting with the following diffs:

• __cxa_new_handler
• __cxa_terminate_handler
• __cxa_unexpected_handler
• _ZNKSt3120vector_base_commonILb1EE20__throw_length_errorEv
• _ZNKSt3121basic_string_commonILb1EE20__throw_length_errorEv
• _ZNKSt3121basic_string_commonILb1EE20__throw_out_of_rangeEv

The last three removed on Linux is more of a bug fix as they are supposed to be provided either by libc++ or inlined.

Its unclear whether the diffs are part of the public ABI or not.

[compnerd]

Okay, so the differences are caused by the data definitions which hold the current allocator (std::set_new_handler), terminator (std::set_terminate), and unexpected exception handler (std::set_unexpected). We could either move those three functions in libc++abi (ABI break) or export the data symbols :-(. Doing so however means that users can directly (and therefore unsafely!) access the pointer in a multithreaded environment. The accessors in the std namespace are just atomic accessors to the pointers. Ill upload a new version of the patch which marks the data symbols for export for now.

[compnerd]

Hmm, actually, investigating this further, it seems that libc++abi already has an implementation of std::set_terminate, std::set_unexpected, std::set_new_handler, std::get_new_handler. Why not add std::get_terminate, std::get_unexpected and not expose the storage? Im not sure how the bindings for these currently works.

I don't believe OS X defaults to two-level namespaces, so the binding there should still resolve if we were to move these. ELF doesn't support two-level namespaces, so again, we should be safe there. That leaves COFF, which I don't believe has a stable ABI, so we should be able to get away with this for now.

Furthermore, to support this move, there is prior art supporting this model. libcxxrt and libsupc++ both provide these interfaces.

[EricWF]

So the first three symbols mentioned need to be reexported. See https://llvm.org/bugs/show_bug.cgi?id=23238 for a related bug.
They are listed as part of the spec in "spec.html" and we shouldn't change that now.
Better safe than sorry especially when managing the ABI.

You noted that __cxa_throw_bad_array_new_length and __cxa_uncaught_exceptions are added on OS X, but that is true before this change as well (when comparing to the system library).

I'm not really concerned with the basic_string methods that go away. The symbols are also exported by libc++.so and so they should always be present anyway.

[EricWF]

Also the title of this revision is the opposite of what it does.

[EricWF]

Added cfe-commits.

[compnerd]

SVN r270816 with the changes.

CMakeLists.txt

 
 append_if(LIBCXXABI_C_FLAGS LIBCXXABI_HAS_FUNWIND_TABLES -funwind-tables)
 
+append_if(LIBCXXABI_COMPILE_FLAGS LIBCXXABI_HAS_FVISIBILITY_HIDDEN_FLAG -fvisibility=hidden)
+
 # Assert
 string(TOUPPER "${CMAKE_BUILD_TYPE}" uppercase_CMAKE_BUILD_TYPE)
 if (LIBCXXABI_ENABLE_ASSERTIONS)

cmake/config-ix.cmake

 check_cxx_compiler_flag(-fno-exceptions       LIBCXXABI_HAS_NO_EXCEPTIONS_FLAG)
 check_cxx_compiler_flag(-fno-rtti             LIBCXXABI_HAS_NO_RTTI_FLAG)
 check_cxx_compiler_flag(-fstrict-aliasing     LIBCXXABI_HAS_FSTRICT_ALIASING_FLAG)
+check_cxx_compiler_flag(-fvisibility=hidden   LIBCXXABI_HAS_FVISIBILITY_HIDDEN_FLAG)
 check_cxx_compiler_flag(-nodefaultlibs        LIBCXXABI_HAS_NODEFAULTLIBS_FLAG)
 check_cxx_compiler_flag(-nostdinc++           LIBCXXABI_HAS_NOSTDINCXX_FLAG)
 check_cxx_compiler_flag(-Wall                 LIBCXXABI_HAS_WALL_FLAG)